The present invention is directed to a circuit for monitoring a plurality of analog quantities and for signaling the exceeding of these quantities of threshold values utilizing a differential amplifier. A reference voltage source is connected between a grounded potential and one input of the differential amplifier and a reference resistor is connected between the grounded potential and the other input of the differential amplifier. Quantities to be monitored are represented by voltages or by impressed currents that are supplied to the input of the differential amplifier which is connected to the reference resistor. The representative voltages or impressed currents are supplied via decoupling diodes and coupling resistors.
The principle regarding the monitoring of threshold values by prior art differential amplifiers is set forth below with reference to FIG. 1 of the drawings.
The quantity to be monitored is represented by a voltage of a voltage source U, this voltage source U being connected via a coupling resistor R and, for proper operating conditions, via a diode D to one input of a differential amplifier V that is additionally connected to a reference resistor RO. A reference voltage source UO is connected to the other input of the differential amplifier V. This reference voltage source UO, as well as the reference resistor RO, has its second terminal connected to the grounded potential. Together with the reference resistor RO, the coupling reistor R forms a voltage divider that is fed by a voltage U' that is equal to the voltage U less the voltage drop across the diode D, this voltage U representing the quantity to be monitored. As soon as this voltage U exceeds the prescribed threshold value, the voltage drop across the reference resistor RO also rises above the prescribed value of the reference voltge UO. As a result, the differential amplifier V signals the exceeding of the threshold value in a well known manner.
In an apparatus wherein a plurality of parameters and operating values that are usually represented by analog voltages or by impressed currents are to be monitord, a separate monitoring means is usually provided for each of these quantities. This, however, proves to be extremely involved. This problem has special significance in apparatus that function with a plurality of supply voltages, indirect voltages, output voltages, analog input and output signals, as well as incoming and outgoing binary messages that are to be monitored for exceeding threshold values or levels.
A prior art circuit is shown in FIG. 2 which utilizes a single differential amplifier for monitoring a plurality of such quantities.
Two voltages (or an arbitrary number of voltages) such as voltage sources U1, U2 are to be monitored for exceeding a threshold value and are connected to voltage dividers R1, R0 and respectively R2, R'0 whose center taps are connected to an input of the differential amplifier V via decoupling diodes D1, D2 whereby the resistors R0, and R'0 are reference resistors. Although this circuit is adequate with a single differential amplifier V and with a single reference voltage source U0, two resistors for each quantity to be monitored are still required.